The present invention relates to optical scanners and more specifically to a laser diode alignment apparatus and method for optical scanners.
Optical scanners are well known for their usefulness in retail checkout and inventory control. Optical scanners generally employ a laser diode, the light from which is focused and collimated to produce a scanning beam. An optical transceiver directs the beam against a plurality of stationary mirrors, and collects the beam after it is reflected by a bar code label. A motor rotates the optical transceiver, and a detector receives the returning beam. The pattern produced by such a scanner is characterized by lines oriented at various angles to one another. An example of an optical scanner can be found in commonly assigned U.S. Pat. No. 4,971,410, entitled, "Scanning and Collection System for a Compact Laser", issued Nov. 20, 1990, to Wike, Jr. et al. This patent is hereby incorporated by reference.
The laser diodes used in optical scanners must be aligned. Misalignment is measured as "centering" error and "pointing" error. Centering error is defined as lateral deviation from an optimum laser beam exit point on the laser mount. Centering error in scanners similar to the one disclosed in Wike, Jr., results in the laser beam being parallel to the motor shaft but not along the centerline of the shaft. Thus, the beam center strikes the wrong point on a deflecting mirror at the end of the shaft.
Pointing error is defined as angular deviation from the centerline of the shaft. Pointing error also results in the laser beam striking the deflecting mirror at the wrong location. If the pointing error is large enough, the beam will reflect off the interior wall of the shaft. Generally, optical scanners are more sensitive to pointing error than to centering error. A very small amount of pointing error will result in a large loss of beam power.
Known methods for eliminating centering and pointing error involve positioning a multitude of mirrors and making a multitude of tedious back-and-forth adjustments. These methods are complicated by the fact that centering adjustments are often undone by pointing adjustments, and vise versa.
Therefore, it would be desirable to produce an optical scanner which incorporates an apparatus for minimizing centering and pointing error. It would also be desirable for such an apparatus to be capable of minimizing both types of error using minimal adjustment.